"Vergangenheit" is German for "past" as in "background, yesteryear, foretime, and also past tense in a grammatical sense."
Scenes in this episode between Victoria Hamilton (Queen Mother) and Jared Harris (King George VI) were actually filmed during production of season one as Harris would no longer be available for filming in the second season.
Rudolf Hess (Paul A Munday), one of the Nazis featured in the episode, was one of Adolf Hitler's most devoted men, and personally wrote down Hitler's book "Mein Kampf" as it was dictated to him while both were imprisoned at Landsberg Prison. Hess served as Deputy Führer from 1933 to 1941, when he flew solo to Scotland in an attempt to negotiate peace with the United Kingdom. He was taken prisoner and eventually convicted of crimes against peace, serving a life sentence until his suicide in 1987.
Despite Queen Elizabeth condemning the Duke of Windsor, he was never banished from the royal family or the United Kingdom upon the release of the Marburg Files. His public appearances continued until 1968, when he attended Princess Marina's (later Duchess of Kent, his sister-in-law) funeral at Royal Burial Ground, Frogmore (where he is also buried).
Stella Gonet, who plays Historian Margaret Lambert, would later go on to play Queen Elizabeth II in Spencer (2021). The film is inspired by Diana, the Princess of Wales' (portrayed by Kristen Stewart ) divorce to Charles, Prince of Wales (portrayed by Jack Farthing ) and depicts Diana's life leading up to the divorce.